Hybrid and monolithic integration of planar lightwave circuits (PLCs)

While silica waveguide PLC products have been deployed in various systems and applications, hybrid integration of semiconductor opto-electronic devices on silica-based planar lightwave circuit (PLC) has become the mainstream platform for small form factor, low-cost and high volume integrated transceiver modules. One of the main benefits of hybrid integration is the wafer-scale process, which greatly reduces chip/module size and assembly cost. This paper reviews the development of this technology, and as an example, presents a hybrid integrated transmitter with four wavelengths on silica PLC chip for LX4 and 10GbE applications.

Integrated photonic platforms such as the SOI platform, the InP platform and the Si3N4 platforms have their own advantages and limitations. To achieve more diverse functionalities on a photonic chip, the combination of different material platforms is extremely important. This is generally carried out by either hybrid integration, heterogeneous integration or monolithic integration. The goal of this thesis is to develop generic integrated platforms and building blocks that enable various active-passive photonic integrations based on the Si3N4 platform. Chapter 1 reviewed the challenges, integration schemes in photonic integrated circuits (PIC), and recent advances in the Si3N4 PICs. Benefiting from excellent optical features of the Si3N4, i.e., the low-loss and large transparency window (0.4–2.35 µm), in Chapter 2, we investigated passive building blocks with optical designs, providing favorable performance of individual components in the Si3N4 platform. Then in Chapter 3, we demonstrated the hybrid integration and monolithic integration between the Si3N4 and polymer material with high-performance optical couplings. In order to realize the active-passive integration in the Si3N4 platform, in Chapter 4, we developed a double-layer photonic platform with standardized processes and good overlay by monolithic integration between the Si3N4 and rare-earth-ion doped Al2O3 material, showing great potentials of scalable and tolerant integrations of active functionalities in the Si3N4 platform. Chapter 5 demonstrated the first application of the double-layer platform, i.e., the integrated Al2O3:Er3+-Si3N4 amplifiers. A novel gain characterization methodology is introduced and high optical gain (~18 dB) has been achieved. Overall, the work in this thesis provides advances in understanding the characteristics of different integration platforms, especially in the development of the double-layer platforms for the active-passive integration of the Si3N4 and rare-earth-ion doped Al2O3, and the integrated Al2O3:Er3+-Si3N4 amplifiers. The investigations pave the way of various applications based on active-passive integration of the Si3N4 and rare-earth-ion based materials.

Integrated Optical Circuits (IOCs) have been under development for over three decades. In an analogy to electronic integrated circuits, developers of IOCs envision the possibility of combining several or many optical processing functions on a single miniature platform, such as a semiconductor chip, fabricated using processes similar to those used for electronic chip production. Also called Planar Lightwave Circuits (PLCs) or Planar Optical Chips (POCs), these devices incorporate functional optical components such as linear or curved waveguides to conduct light from one location to another, filters fabricated from specially shaped waveguides that control the spectral characteristics of the light, and lenses and mirrors embedded in waveguides to alter the shape of the light. The PLCs may be interfaced to other optical components and devices via optical fibers or other interconnecting devices such as the mode converters described below.

To develop polarization-insensitive semiconductor optical amplifier (SOA) gates into more efficient and functional optical components by using hybrid integration with a planer lightwave circuit (PLC) platform, vertical tapered spot-size converters were integrated into a SOA gate with a narrow mesa-stripe structure of submicrometer width leading to low coupling loss (2.6 dB). Very-low polarization dependence of gain ( 30 dB) were achieved in a wide wavelength range (40 nm). High-speed (<1 us) gate operation was also demonstrated. These results indicate that the spot-size converted SOA integrated on a PLC platform will perform well in wavelength-division-multiplexing (WDM) applications.

Wavelength division multiplexing (WDM) systems are widely deployed in high-capacity long-haul transmission systems. They can provide cost-effective additional capacity on fibers overloaded by the increase in data traffic. Current WDM systems are used mainly for point-to-point transmission links. Next generation WDM technologies are expected to provide full functionality at both the system and network levels. Next generation WDM system will require many optical components, including optical filters, add/drop multiplexers, circuit switches and wavelength converters. Optical components using planar lightwave circuits (PLCs) are now widely used in WDM systems. Arrayed waveguide gratings (AWGs), in particular, play an important role in current WDM systems. In the next generation system, optical components using PLCs are expected to provide many functions that counterparts cannot. In this paper, we will describe the current and future WDM components using PLCs, focusing on AWGs and switchable add/drop filters.

Hybrid integration of III–V and ferroelectric materials is being broadly adopted to enhance functionalities in silicon photonic integrated circuits (PICs). Bonding and transfer printing have been the popular approaches for integration of III–V gain media with silicon PICs. Similar approaches are also being considered for ferroelectrics to enable larger RF modulation bandwidths, higher linearity, lower optical loss integrated optical modulators on chip. In this paper, we review existing integration strategies of III–V materials and present a route towards hybrid integration of both III–V and ferroelectrics on the same chip. We show that adiabatic transformation of the optical mode between hybrid ferroelectric and silicon sections enables efficient transfer of optical modal energies for maximum overlap of the optical mode with the ferroelectric media, similar to approaches adopted to maximize optical overlap with the gain section, thereby reducing lasing thresholds for hybrid III–V integration with silicon PICs. Preliminary designs are presented to enable a foundry compatible hybrid integration route of diverse functionalities on silicon PICs.

The large refractive index contrast between silicon nitride and silicon dioxide allows silicon nitride/dioxide planar waveguides to have a small mode size and low radiation bending loss compared with doped silicon dioxide waveguides. Small waveguide bend with low radiation loss can help make small integrated planar lightwave circuits (PLCs), and also high-Q waveguide ring resonators. This presentation will talk about the design, fabrication and characterization of low loss silicon nitride/dioxide planar waveguide devices including waveguide bend, waveguide cross, and leaky mode waveguide polarizer. The key contribution of this work is the use of the lateral mode interference (LMI) 3dB splitter to accurately measure the loss of the planar lightwave circuit devices. We will also talk about the waveguide ring resonators with silicon nitride/dioxide materials. The application for photonic biochemical sensors will also be discussed.

A novel fabrication method and structure of a planar lightwave circuit (PLC) platform for hybrid integration of an optical module by passive alignment technology is presented. Precise formation of V-grooves in the PLC platform can be easily obtained by the proposed process. The passive alignments of optical elements, including optical fiber, are achieved in one-mask. LD modules were implemented by the proposed method.

This paper describes hybrid assembly of a wavelength selective switch using laser-formed glass bumps. Recently, a process was developed for forming raised bumps on IR-absorbing glass substrates using a focused laser beam. Glass bumps with heights exceeding 90 m have been formed with an accuracy of ~ 100 nm using multiple laser shots. Proper selection of materials permits the bump height to be raised or lowered via subsequent laser shots by adjusting laser power. Processes are described for precision alignment of planar AWG components to a pedestal-mounted planar SOA array by forming three glass laser bumps beneath the AWG components. While the iterative process of bump formation and component position assessment was perf ormed manually, this work demonstrates that the process is predictable and well-suited for automation. Keywords: Hybrid assembly, hybrid integration, laser bumps, wavelength-selective switch, planar lightwave circuit, optical waveguide alignment, precision array alignment, automated alignment

This presentation will describe the Air Force Research Laboratory Highly Integrated Photonics Program (AF HIP) and its objective to integrate on a monolithic device, all of the optical components required to serve as a bus coupler in an all optical data communication network. This research and development program utilizes advanced technologies in silicon on insulator (SOI) and silica planar lightwave circuits (PLC) to design, develop, characterize, and demonstrate highly integrated photonic devices that can be transitioned into both current and emerging tactical platforms for the U.S. Air Force. This effort strives to overcome several existing constraints with respect to the integration and packaging aspects of the current generation of COTS optical devices. Monolithic integration (chips fabricated out of a single material system) remains the ultimate vision for integrated optics.

In this work, the guided modes of a photonic crystal polarization beam splitter (PC-PBS) are studied. We demonstrate that the transmission of a low-loss photonic crystal 120° waveguide bend integrated with the PBS will be influenced if the PBS is multi-moded. We propose a single-moded PC-PBS structure by introducing deformed structures, and it shows twice the enhancement of the transmission. This device with remarkable improvement of performance is promising in the use of photonic crystal integrated circuits design.

The goal of integrated quantum photonics is to combine components for the generation, manipulation, and detection of nonclassical light in a phase-stable and efficient platform. Solid-state quantum emitters have recently reached outstanding performance as single-photon sources. In parallel, photonic integrated circuits have been advanced to the point that thousands of components can be controlled on a chip with high efficiency and phase stability. Consequently, researchers are now beginning to combine these leading quantum emitters and photonic integrated circuit platforms to realize the best properties of each technology. In this paper, we review recent advances in integrated quantum photonics based on such hybrid systems. Although hybrid integration solves many limitations of individual platforms, it also introduces new challenges that arise from interfacing different materials. We review various issues in solid-state quantum emitters and photonic integrated circuits, the hybrid integration techniques that bridge these two systems, and methods for chip-based manipulation of photons and emitters. Finally, we discuss the remaining challenges and future prospects of on-chip quantum photonics with integrated quantum emitters.

We propose a novel three-dimensional (3D) monolithic optoelectronic integration platform. Such platform integrates both electrical and photonic devices in a bulk silicon wafer, which eliminates the high-cost silicon-on-insulator (SOI) wafer and is more suitable for process requirements of electronic and photonic integrated circuits (ICs). For proof-of-concept, we demonstrate a three-dimensional photodetector and WDM receiver system. The Ge is grown on a 8-inch bulk silicon wafer while the optical waveguide is defined in a SiN layer which is deposited on top of it, with ~4 µm oxide sandwiched in between. The light is directed to the Ge photodetector from the SiN waveguide vertically by using grating coupler with a Aluminum mirror on top of it. The measured photodetector responsivity is ~0.2 A/W and the 3-dB bandwidth is ~2 GHz. Using such vertical-coupled photodetector, we demonstrated an 8-channel receiver by integrating a 1 × 8 arrayed waveguide grating (AWG). High-quality optical signal detection with up to 10 Gbit/s data rate is demonstrated, suggesting a 80 Gbit/s throughput. Such receiver can be applied to on-chip optical interconnect, DRAM interface, and telecommunication systems.

A silica micro-lens has been proposed which can be integrated with planar optical waveguide circuits. The lens pair enables an optical signal to travel in free space between two opposing planar waveguides with minimal optical loss. Each lens pair consists of a slab GRIN lens with a convexly shaped front face. This paper brieftly reviews the micro-lens design process and reports progress in fabircating the device. The characterisation of the GRIN layer and masking experiments used to evaluate the deep oxide etch are presented. A selectivity of 250:1 was achieved for the deep oxide etch using a NiCr mask.

Since the growth of fiber optic communications, extremely low cost optical modules have been expected to be realized. Particularly for the subscriber applications and local area networks (LANs), less than half of past module cost is required. In accordance with the spread of fiber optic transmission, higher volume manufacture is also needed. To attain both of low cost and high productivity, we developed passive alignment technique and optical coupling structure that is suitable for automatic assembly. We also developed automatic assemble equipment for producing several types of optical modules. To reduce optical module cost, it is important to cut down the assembly cost and expensive parts such as optical lens. The passive alignment technique brings us to realize easy assembly of modules and minimize the constructed components of optical coupling. Our mass production techniques include not only for optical coupling but also packaging of the module. We have several types of optical modules according to different fiber communication systems; there are laser modules, detector modules, optical interface modules that are integrated optical coupling and electrical circuit, and PLC (Planar Lightwave Circuit) modules that are integrated passive component. Basically all modules have same optical coupling structure and keep the design rules for automatic assembly. Accordingly, we can use the same equipment for the same assembly process. In this paper, we will introduce basic assembly process and our mass production line for optical coupling and packaging by using passive alignment technique.